This invention relates in general to color television receivers and in particular to means in color television receivers for automatically demagnetizing the color picture tube each time the receiver is energized from a "cold" start.
Nearly all modern color television receivers include some form of automatic degaussing circuitry for subjecting the color television picture to a high, initial amplitude, decaying alternating magnetic field for removing any residual magnetism which may be present in the permeable elements in the picture tube. As is well-known, residual magnetism has a detrimental effect on the color display of the picture tube. Such circuit arrangements generally comprise means for supplying a large initial amplitude, rapidly tapering alternating current to one or more degaussing coils located closely adjacent to the picture tube.
The simplest form of degaussing circuit includes a thermal element, exhibiting a low resistance when cold and a high resistance when hot, for tapering the current. Upon energization of the television receiver, maximum current flows through the degaussing coils because of the low resistance of the thermal element. The current flow rapidly raises the temperature of the thermal element, resulting in an increase in its resistance and consequent reduction of the current. This progressive reduction or tapering of the current gives rise to a similarly tapered alternating magnetic field about the picture tube, which removes any residual magnetism present in its permeable elements.
The amount of degaussing action obtained is, of course, a function of the magnetic field strength, which, in turn, is the product of the number of turns in the degaussing coils and the current through them. With conventional transformers a high initial current is readily obtainable and assures a sufficiently strong magnetic field to adequately degauss the tube. However, in receivers having voltage regulating transformers it is difficult to obtain a sufficiently intense magnetic field because the transformer tends to be current limiting. One solution is to place the degaussing circuit directly across the AC line. While such an arrangement solves the degaussing problem, it poses some safety problems because the degaussing coils (situated near the picture tube) are at line potential. In practice, an additional winding, magnetically coupled to the primary side of the transformer, is used to supply the degaussing current. This solution entails added cost in transformer manufacture and doesn't eliminate the possibility of line potential being present on the receiver chassis in the event of a breakdown between the transformer primary winding and the additional winding.
An example of the latter solution is shown in U.S. Pat. No. 3,798,493 issued Mar. 19, 1974 to Hans E. Manske and assigned to Zenith Radio Corporation. In that patent a separate degaussing winding is wound on the primary side of the voltage regulating transformer and is utilized to supply a degaussing circuit consisting of the series connection of a positive temperature coefficient resistance and a pair of degaussing coils.
Degaussing from the secondary side of a voltage regulating transformer is difficult because of the limited degaussing current available. In all situations, the size of the transformer and its configuration (number of windings and taps) and the amount of "copper" in the degaussing coils are important considerations in providing the consumer with a quality performing receiver at reasonable cost.
The above-mentioned copending application provides for degaussing of the color television receiver picture tube from the secondary side of a voltage regulating transformer. An additional winding, magnetically coupled to the primary winding or directly coupled to the AC line, is positioned on the shunted side of the core and therefore is magnetically coupled to the other secondary windings. This additional winding in conjunction with the secondary windings forms a conventional transformer.
The additional winding is coupled to the primary power source through a thermally responsive element distinct from the degaussing thermal element which effectively "disconnects" the additional winding after it reaches a certain temperature. Consequently, when the receiver is energized from a cold start the thermally responsive element manifests a low resistance and the additional winding is supplied with full primary power. The voltage regulating transformer functions as a conventional transformer and the normally regulated secondary windings provide degaussing current to the degaussing coil. The effect on the other secondary supplied circuitry is minimal. As the thermally responsive element heats up its resistance increases and restores power to the additional winding. Equilibrium is reached at a current value low enough not to interfere with normal regulated operation of the transformer.